Methods and circuits for recovering sync signals from a video signal are known. Such methods and circuits may be adapted for recovering the sync signals sequentially or simultaneously with active processing and decoding of the video signal. However, entire recovery of the sync signals sequentially with the active processing and decoding of the video signal, in general, militates against fast lock-in of the video signal. Accordingly, in order to minimise lock-In time, it is known to pass a video signal through two circuits in parallel, one of which carries out active signal processing and decoding of the video signal, and the other circuit which recovers the sync signals, simultaneously with the processing of the video signal.
Video signal characteristics can vary significantly from one video signal to another, and indeed, within the same video signal. For example, in a nominal one volt video signal, the horizontal sync signal amplitude from the blanking level is nominally 300 mV. However, the amplitude of the horizontal sync signal can range from 10% to 200% of the nominal value of 300 mV. Additionally, signals with low signal to noise ratio which may be received, for example, from a radio frequency (RF) tuner may have noise levels which can mask the horizontal sync signals. Additionally, video signals may be AC coupled or DC coupled, and in the case of AC coupled video signals the DC voltage level of the video signal may vary significantly with the video content. Another problem with video signals, particularly those provided from mechanical recording devices, such as VCR devices, is that the video signal may suffer from several different defects, such as significant steps in the DC voltage level between fields, and indeed, false or missing sync signals may occur from time to time.
One method which is commonly employed for recovering sync signals, requires comparing the video signal with a slice level signal. The value of the slice level signal is generally selected to be midway between the expected value of the blanking level of the video signal and the expected value of the sync tip level of the horizontal sync signal of the video signal.
Some such methods use a fixed slice level, others adjust the slice level in response to the detected amplitude or noise level of the input video signal. However, known methods for setting the slice level require directly measuring the values of the sync tip level and back porch sections of the video signal. These methods require detection of the rising and falling edges of the horizontal sync signal, which in general is carried out by monitoring for a reasonably large positive or negative edge of the video signal, and setting a measuring window for a time thereafter during which the signal level is sampled or accumulated. Once the location of the rising and falling edges of the horizontal sync signal have been determined, the values of the blanking level of the video signal and the sync tip level of the horizontal sync signal can then be measured, and the appropriate setting for the slice level signal can be computed from the values of the blanking level and the sync tip level of the video signal. However, this type of method suffers from the disadvantage that the active region of a video signal may include shapes which replicate a horizontal sync signal, and the detection of such shapes would result in the erroneous determination of the location of the horizontal sync signal, and in turn the erroneous determination of the values of the blanking level and sync tip level of the video signal.
U.S. Pat. No. 6,271,889 of Böhm discloses a method for overcoming this problem. However, the method of Böhm requires that the sync tip levels of the horizontal sync signal of the input video signal should be within a relatively narrow range. Accordingly, the method of Böhm is unsuitable where the amplitude of the horizontal sync signal varies widely. The method of Böhm requires that the input video signal is differentiated in order to locate the rising and falling edges of the horizontal sync signal. The results of the differentiation of the video signal are compared with a positive threshold for determining a valid rising edge, and a negative threshold for determining a valid falling edge. However, these thresholds must be large enough to reject edges caused by noise, and small enough to accept edges representative of the likely range in the amplitudes of the horizontal sync signals in the video signal. Since the horizontal sync signals in some video signals may have been clipped or reduced to the extent that the amplitude of the horizontal sync signal is as low as ten percent of its nominal value, and thus the lowest expected amplitude of the horizontal sync signal of the input video signal may be of an order of magnitude less than the highest expected amplitude of the horizontal sync signal. Accordingly, a serious conflict arises between the chosen thresholds for edge detection in the method of Böhm, and thus, the range of amplitudes of horizontal sync signals of a video signal with which the method of Böhm can operate lies within the range of the nominal amplitude and approximately fifty percent of the nominal amplitude of the horizontal sync signal.
Accordingly, there is a need for a method and a circuit for deriving a synchronisation signal from a video signal, and in particular, there is need for a method and a circuit for deriving a synchronisation signal from a video signal which permits recovery of the horizontal sync signal from the video signal which overcomes the problems of known methods and circuits, and in particular, which facilitates relatively rapid lock-in of the video signal.
The present invention is directed towards providing such a method and a circuit.